As a prior art quality inspection device for a printed matter, there has been proposed, for example, one which is shown in FIG. 7.
This is an example of this device applied to a sheetfeed printing machine having a plate cylinder 1 around which a printing plate is wound, a blanket cylinder 2 around which a rubber blanket is wound, and an impression cylinder 3 to apply a pressure, wherein an image is transferred from the printing plate to the rubber blanket and then printed on printing paper.
This device mainly comprises a camera unit 4 as a detector to take in the surface condition as image data of the printed paper fed to the impression cylinder 3, a lighting unit 5 to illuminate the printed paper surface with high-intensity light, a synchronization device 6 such as a rotary encoder disposed at an end of a shaft of the impression cylinder 3 to obtain a synchronizing (registration) signal in the feed direction of the printed paper discharged with the rotation of the printing machine, and a control unit 7 which is inputted with signals from the camera unit 4 and the synchronization device 6. In the control unit 7, reference image data of a correctly printed paper surface taken in by the camera unit 4 at the beginning of the printing operation is compared with image data of a printed paper surface to be checked taken in by the camera unit 4 afterward during the printing operation synchronizing with the signal from the synchronization device 6 to determine whether or not the difference in level between both data is within a predetermined tolerable range, thereby checking the quality of printing. When the control unit 7 determines the presence of a defect print such as an oil stain, the position of the defect is displayed on a CRT display, an alarm is sounded, and a tape inserter 8 or the like is operated to make a marking for identification of the defective print in a subsequent step.
In FIG. 7, numeral 9 indicates an air blow type paper holding device, and numeral 10 indicates a paper discharge cylinder.
During the printing operation, the printing speed may be increased or decreased as needed for stability of the printing density, condition of paper at the paper feed or paper discharge unit, or in view of production control. In such a case, due to the characteristics of the camera unit 4 of the quality inspection device for printed matters, since the amount of light received by the light receiving element is varied, which results in errors in the determination of quality of the printed matters, a correction for speed is made to maintain the level of received light regardless of such variations in printing speed.
As an example of such correction for speed, there has heretofore been proposed a method in which the gain for the image signals from the light receiving element is controlled by an AGC circuit, but this method alone is too narrow in the range of correction compared to the range of variations in printing speed.